The present invention relates to a step automatic transmission employed as a transmission for vehicle.
Japanese Patent Application Publication No. 2004-176765 or U.S. Pat. No. 6,648,791 (FIGS. 23 to 26) discloses an automatic transmission adapted to achieve forward seven speed-ratios by using three planetary gear sets. In the automatic transmission disclosed in the above Japanese Patent Application, the forward seven speed-ratios are obtained by using six friction elements and three single-pinion-type planetary gear sets. This single-pinion-type planetary gear set has an advantage in transfer efficiency and gear noise and also an advantage in durability because of the nonnecessity for reducing a diameter of pinion gear. Moreover similarly, the automatic transmission disclosed in the above United States Patent achieves forward six speed-ratios to forward eight speed-ratios by using five friction elements and three single-pinion-type planetary gear sets.